Hm, 11 voices sounds weird. But this is really what carlinux said. The dumped audio package (region 0x11) is called "Seminole Audio Pkg 1" and it's only 570KB long. It's doubtful it contains all the 11 voices.
I'm primarily afraid of EU firmware being not fully compatible with US hardware and US NVRAM contents. This has to be tested.
And bad voice data could also make firmware crash, which could be fatal with no pre-boot mode. So this has to be tested also.
Well, I'm probably more paranoid this time than you guys are, but just think about it... no good pre-boot mode, no way to recover from firmware crashes.
I don't know why Garmin makes such devices.